A current mirror is a circuit that copies or “mirrors” a reference current in one active device by controlling a current in another active device. The current mirror may function as a current source or a current sink. Current mirrors are often used to provide bias currents or to serve as an active load.
An ideal current mirror has an infinite output resistance that is independent of voltage. In practice, however, the output resistance is finite. In addition, a functional current mirror requires a voltage drop across its input and output stages. The size of the required voltage drop limits one or more of the input current range, the output current range, or the size of the load being driven. The required voltage drop is an overhead that limits the signal swing available for the input or output or both. The required voltage drop becomes increasingly important as the supply level is reduced.